Recently, for example, JP 2007-113548A discloses an apparatus that detects a fuel leak in a leak fuel system to return a leak fuel to a fuel tank from an internal combustion engine. In this apparatus, in a case where a temperature detected by a temperature sensor arranged in the leak fuel system is decreased, for example, to as low a value as an outside air temperature, it is determined that a fuel leak from the leak fuel system to the outside is caused.
However, this device is so constructed as to measure the temperature of the fuel at only one specified position of the leak fuel system. Hence, the device can diagnose a fuel leak only in a state where the leak fuel stably flows in the leak fuel system in a continuous steady operating state of the internal combustion engine. In addition, in a case where the fuel flowing in the leak fuel system is a liquid fuel, as far as a serious fuel leak of a degree in which the leak fuel reaching the temperature sensor is lost is not caused, a clear decrease in the temperature is hard to occur. For this reason, in the construction described above, it is difficult to detect a small amount of fuel leak.